Method for purifying trimethylolpropane, which is produced by hydrogenation, by means of continuous distillation

ABSTRACT

A process is disclosed for the purification, by distillation, of trimethylolpropane originating from the hydrogenation of 2,2-dimethylolbutanal, said process comprising the following steps:  
     (a) reaction of n-butyraldehyde with formaldehyde in the presence of catalytic amounts of a tertiary amine, and hydrogenation of the resulting mixture to give a mixture containing trimethylolpropane;  
     (b) separation of water, methanol, trialkylamine and/or trialkylammonium formate by distillation;  
     (c) heating of the residue obtained in (b) under reduced pressure to a temperature at which TMP is volatile and compounds boiling above TMP are cleaved, in order to separate off, by distillation, TMP and compounds more volatile than TMP;  
     (d) distillation of the distillate obtained in (c) in order to separate off the more volatile compounds and recover pure TMP; and  
     (e) optional distillation of the TMP obtained in (d) in order to recover TMP with a low APHA color index. A process is also disclosed in which trialkylammonium formate is distilled under mild conditions from crude mixtures of polyhydric alcohols, predominantly tri-methylolpropane.

[0001] The present invention relates to the field of industrialchemistry. More precisely, it relates to a process in whichtrimethylolpropane obtained by the hydrogenation of dimethylolbutanal ispurified by distillation. The present invention further relates to aprocess in which trialkylammonium formate produced as a by-product inthe preparation of the alkylolated alkanal, from the trialkylamine usedas catalyst and formic acid, can be separated off by simpledistillation.

[0002] Trimethylolpropane, hereafter abbreviated to TMP, is a trihydricalcohol which has become widely used in the manufacture of surfacecoatings, polyurethanes and polyesters, for example alkyd resins.Trimethylolpropane is prepared by means of a condensation reactionbetween n-butyraldehyde and formaldehyde. This reaction can be carriedout using different process variants.

[0003] Firstly, there is the so-called Cannizzaro process, in which thebutyraldehyde is reacted with the formaldehyde in the presence ofstoichiometric amounts of a base. 2,2-Dimethylolbutanal is formed in thefirst step and then reacts with excess formaldehyde, in a so-calledcrossed Cannizzaro reaction, to give formic acid and trimethylolpropane.The Cannizzaro process is referred to as inorganic or organic accordingto the type of base. The inorganic procedure employs an inorganic base,usually NaOH or Ca(OH)2. The disadvantage of this procedure is that aquantity of unwanted by-products is formed which are difficult toseparate off and which interfere with the subsequent use of thetrimethylolpropane. Also, one mole equivalent of formic acid salt isformed; this has to be discarded, thereby increasing the consumption offormaldehyde and contributing to environmental pollution.

[0004] In the organic Cannizzaro process, a tertiary amine, generally atrialkylamine, is used in place of the inorganic base. The reactionproceeds as described above to give one equivalent of the ammoniumformate of the amine in question. This can be worked up further byappropriate measures, whereby at least the amine can be recovered andrecycled into the reaction. The crude TMP obtained can be worked up topure TMP in a variety of ways.

[0005] One further development is the hydrogenation process, in whichbutyraldehyde and formaldehyde are reacted together in the presence ofcatalytic amounts of a tertiary amnine, generally approx. 5 to 10 mol %,rather than at least stoichiometric amounts. In this case the reactionstops at the 2,2-dimethylolbutanal stage and this compound is thenconverted to trimethylolpropane by hydrogenation. This method does notgive rise to stoichiometric amounts of a formate, and the resultingsolution is easier to purify because fewer interfering by-products areformed. In many cases, however, it is necessary to resort to reactionengineering measures to achieve a complete conversion of the educts todimethylolbutanal. The description of an efficient process can be foundin WO 98/28253 in the name of the Applicant.

[0006] The state of the art contains a wealth of publications describingdifferent techniques for working up trimethylolpropane. A study of thesepublications clearly reveals the differences in work-up demanded by thedifferent ways of preparing trimethylolpropane. The followingpublications relate to the purification of trimethylolpropane obtainedby the inorganic Cannizzaro process.

[0007] DD-P-45 078 discloses a process in which the crude TMP obtainedis treated with a secondary cycloaliphatic alcohol, for examplecyclohexanol, water is then distilled off azeotropically with thisalcohol and the formates which have precipitated are filtered off. Afterdistillation of the excess alcohol, the crude product obtained is thenpurified by distillation.

[0008] DD-P-287 251 describes a process for freeing TMP of thehigh-boiling components formed. In the vacuum distillation of crude TMP,the high-boiling components accumulate in the fractions less volatilethan TMP. Many of the high-boiling components, which consist ofsecondary reaction products of TMP, especially formals, can be convertedback to TMP by the addition of 0.02 to 0.05 kg acid/kg distillate,thereby increasing the yield.

[0009] Also, GB 1 290 036 describes a process for the decomposition ofTMP formals in crude batches obtained by the inorganic Cannizzaroprocess. By the addition of cation exchange resins and heating, formalscontained in the crude mixtures, which have a similar boiling point toTMP, are converted to products of different boiling points which canreadily be separated off by distillation. Pure TMP can be obtained.

[0010] U.S. Pat. No. 3,097,245 describes a process for the preparationof trimethylolpropane with an APHA color index of between 50 and 200.This color index is achieved by main-taining specific reactionconditions in respect of temperature, reaction time, pH andconcentration of the starting compounds. The reaction is followed bytreatment of the resulting solution with ion exchange resins.

[0011] U.S. Pat. No. 5,603,835 discloses a process for the preparationof TMP with APHA color indices of <100. These are achieved by extractiveaftertreatment of the crude TMP solutions obtained with an ether or anester. The TMP solutions used generally originate from the inorganicCannizzaro process.

[0012] By contrast, crude TMP originating from the organic Cannizzaroprocess is worked up differently.

[0013] DE-P-142 090 describes the work-up of such a crude TMP mixture.This crude mixture is worked up by distillation and then hydrogenatedand distilled again. Such a process is expensive, demands a high vacuumand gives low yields.

[0014] In particular, in the preparation of TMP by the organicCannizzaro process, there is an interfering secondary reaction which canmarkedly reduce the yield of TMP. The trialkylammonium formate producedin the reaction reacts under specific conditions, for exampledehydration of the solution or heating, to give trialkylamine andtrimethylolpropane formates. These compromise the yield oftrimethylolpropane and should therefore be cleaved as completely aspossible without the simultaneous appearance of unwanted secondaryreactions.

[0015] WO 97/17313 discloses a process suitable for this purpose. In thefirst step, trimethylolpropane is prepared in a manner known per se byreacting formaldehyde with butyraldehyde in the presence ofstoichiometric amounts of a tertiary amine. In the second step, thecrude TMP mixture is freed of excess water, tertiary amine andformaldehyde. In the third step, the residual mixture is heated, causingthe cleavage of the trialkylammonium formate into trialkylamine andformic acid (both of which are separated off) and the formation of TMPformates. In the fourth step, the amine which has been separated off isrecycled, either into the first step or into the fifth step whichfollows. In this fifth step, the TMP formate obtained is reacted with alower alcohol (this reaction being catalyzed by the very amine which hasbeen separated off) to free the TMP with the formation of methylformate.

[0016] A similar process is disclosed in DE-A-198 48 568. The crude TMPmixture obtained after the conventional reaction in the presence ofstoichiometric amounts of a trialkylamine is heated, freed oftrialkylamine and, before work-up by distillation, treated with eitherwater, ammonia, a primary amine or a secondary amine. Thetrimethylolpropane formate produced on heating is converted to TMP andformic acid or a formamide. The yield of TMP is increased.

[0017] However, the abovementioned processes are only of limitedsuitability for the efficient work-up of a TMP mixture obtained by theso-called hydrogenation process, in which only catalytic amounts oftrialkylamine are used and which consequently also contains only smallamounts of trialkylammonium formate.

[0018] It is therefore an object of the present invention to providesuch a process. This process should furthermore make it possible toprepare TMP with a high purity, preferably >99%, with a low color indexof 10 to 100 APHA and in high yield.

[0019] We have found that this object is achieved by a process for thepurification, by distillation, of trimethylolpropane originating fromthe hydrogenation of 2,2-dimethylolbutanal, said process comprising thefollowing steps:

[0020] a) reaction of n-butyraldehyde with formaldehyde in the presenceof catalytic amounts of a tertiary amine, and hydrogenation of theresulting mixture to give a mixture containing trimethylolpropane;

[0021] b) separation of water, methanol, trialkylamine and/ortrialkylammonium formate by distillation;

[0022] c) heating of the residue obtained in b) to a temperature atwhich trimethylolpropane is volatile and compounds boiling abovetrimethylolpropane are at least partially cleaved, in order to separateoff, by distillation, trimethylolpropane and compounds more volatilethan trimethylolpropane;

[0023] d) distillation of the distillate obtained in c) in order toseparate off the more volatile compounds and recover puretrimethylolpropane; and

[0024] e) optional distillation of the trimethylolpropane obtained in d)in order to recover TMP with a low APHA color index.

[0025] We have also found that this object is achieved by a generallyapplicable process for removing trialkylammonium formates frompolyhydric alcohols obtained by condensing formaldehyde with a higheraldehyde, wherein the trialkylammonium formate, together with methanol,trialkylamine and optionally water, is distilled from the alcohol atpressures of 20 to 200 mbar and bottom temperatures of 60 to 140° C. andwith short residence times. The polyhydric alcohol is trimethylolpropanein particular.

[0026] Within the framework of the process according to the inventionfor the purification of trimethylolpropane by distillation, crude TMPsolutions which have been prepared by the so-called hydrogenationprocess are purified. In other words, the TMP has been obtained bycondensing n-butyraldehyde with formaldehyde in the presence ofcatalytic amounts of a tertiary amine, this being followed by catalytichydrogenation of the dimethylolbutanal mixture formed. Thus the crudeTMP contains no alkali metal or alkaline earth metal formates or otherimpurities produced in the inorganic Cannizzaro process. Similarly, incontrast to the organic Cannizzaro process, the crude TMP contains onlysmall amounts of approx. 5 to 10 mol % of trialkylammonium formates orfree trialkylamine.

[0027] In addition to trimethylolpropane and water, the crude TMP whichoriginates from hydrogenation and is to be subjected to the purificationprocess according to the invention also contains methanol,trialkylamine, trialkylammonium formate, longer-chain linear andbranched alcohols and diols, for example methylbutanol orethylpropanediol, addition products of formaldehyde and methanol withtrimethylolpropane, acetals, such as dimethylolbutyraldehyde TMP acetal,and so-called di-TMP.

[0028] Good results have been achieved with hydrogenation dischargescontaining 10 to 40% by weight of trimethylolpropane, 0.5 to 5% byweight of methanol, 1 to 6% by weight of methylbutanol, 1 to 10% byweight of trialkylammonium formate, 0 to 5% by weight of2-ethylpropanediol, 2 to 10% by weight of high-boiling components, suchas di-TMP or other addition products, and 5 to 80% by weight of water.Hydrogenation discharges of such a composition can be obtained forexample by the process described in WO 98/28253.

[0029] In process step b) following hydrogenation, the hydrogenationdischarge is then subjected to distillation, in which water and otherreadily volatile compounds, such as methanol, trialkylamine andoptionally trialkylammonium formate, are separated off. Thisdistillation is carried out with the apparatuses familiar to thoseskilled in the art, for example evaporators and/or distillation columns,the pressures used being 20 mbar to 1 bar. The composition of the bottommixture obtained after distillation is very dependent on the conditionsunder which the distillation step has been carried out.

[0030] If this distillation is carried out under mild conditions, thetrialkylammonium formate present in the solution distils off with theother low-boiling components mentioned above, and only small amounts ofTMP formates are produced, if any, as described previously. In thecontext of the present invention, mild conditions are low pressures of<400 mbar, preferably 20 to 200 mbar and particularly preferably 40 to150 mbar, and bottom temperatures of <200° C., preferably 60 to 140° C.and particularly preferably 80 to 120° C. Under these conditions andwhen using appropriate apparatuses, it is possible to achieve the shortresidence times necessary to suppress further reaction of thetrialkylammonium formate. Short residence times according to theinvention range from 5 min to 2 h. Apparatuses which allow such shortresidence times to be observed are, for example, film evaporators,falling film evaporators or spiral tube evaporators. These apparatusescan be operated with or without an attached distillation column.

[0031] Under the conditions described, trialkylammonium formate can beseparated from trimethylolpropane mixtures without a significant degreeof decomposition being observed. Under suitable reaction conditions,approximately at least 95% of the amount of trialkylammonium formatepresent can be distilled off at the top. Thus trimethylolpropane formateis only formed in amounts which do not exceed approximately 5%, based onthe amount of trialkylammonium formate.

[0032] In one variant, the trialkylammonium formate can be separated offin such a way that, in a first step (bi), water is initially separatedfrom the reaction mixture under extremely mild conditions, the waterbeing entrained so as to reduce the residual water content of themixture to below 10% by weight, preferably below 5% by weight. Thedecomposition reaction of the trialkylammonium formate can thereby befurther suppressed. The initial separation of water is then followed, ina second step (bii), by distillation of the other low-boiling componentsand the trialkylammonium formate, as described above.

[0033] The separation of trialkylammonium formate by distillation isparticularly advantageous in the purification of crudetrimethylolpropane mixtures. Of course, this distillative separationaccording to the invention can also be carried out in connection withthe synthesis of other polyhydric alcohols obtained by condensingformaldehyde with higher aldehydes in the presence of trialkylamines. Acondition is that the boiling point of the polyhydric alcohol is not tooclose to that of the trialkylammonium formate, thereby allowing thelatter to be separated off by distillation. Also, of course, thepolyhydric alcohol must not decompose under the conditions used in thedistillation.

[0034] The distillative separation of trialkylammonium formate accordingto the invention is particularly suitable in the case of polyhydricalcohols which have been prepared by the hydrogenation process. Therelatively small amounts of trialkylammonium formate obtained in thisprocess, being at most approx. 10 mol% based on the alcohol, can thus beefficiently separated off and correspondingly small amounts of alcoholformates are produced. However, the process according to the inventioncan also be used for polyhydric alcohols which have been prepared by theorganic Cannizzaro process. The large amounts of trialkylammoniumformate produced in this case frequently demand a greater expenditure onapparatus for the distillation.

[0035] The distillative separation according to the invention can becarried out batchwise or continuously. The evaporator can be operatedwithout or, preferably, with a recycle stream.

[0036] If, on the other hand, water and the other low-boiling componentsare distilled off at pressures of >200 mbar, preferably >400 mbar, andbottom temperatures of >140° C., preferably 160 to 185° C., a reactiontakes place between the trialkylammonium formate and trimethylolpropaneto give trialkylamine and formates of the trimethylolpropane. Theformation of these formates is also favored by long residence times. Ifthis procedure is chosen, the trialkylamine formed distils off with theother low-boiling components and can be re-used in the aldolizationreaction. The bottom product obtained after distillation then containsapprox. 2 to 10% by weight of trimethylolpropane formate.

[0037] If process step (b), i.e. the dehydration of the crude discharge,is carried out under conditions where trimethylolpropane formates areproduced, step (b) is followed by a process step (bb), in which theseformates are cleaved and TMP is recovered.

[0038] This can be effected in a manner known per se, for example bycarrying out a transesterification with a lower alcohol, for examplemethanol, to give formates of this alcohol and TMP. Thistransesterification can be carried out for example as described inEP-A-289 921, in the presence of catalytic amounts of alkali metal oralkaline earth metal alcoholates. The reaction can also be carried outas disclosed in WO 97/17313, where a tertiary amine is used to catalyzethe reaction. The reaction can also be catalyzed by acid.

[0039] Another possible way of freeing trimethylolpropane from itsformates is to react them with an anhydrous secondary amine, asdescribed in the German patent application entitled “Verfahren zurUmwandlung von bei der Trimethylolalkan-Herstellung anfallendenTrimethylolalkanformiat” [“Process for converting trimethylolalkaneformate obtained in the preparation of trimethylolalkane”] (Applicant:BASF AG).

[0040] In process step (c), the residue originating from step (b) or(bb) is then heated to a temperature at which the so-called high-boilingcomponents, i.e. compounds less volatile than TMP, are separated off.This heating takes place under reduced pressures of 5 to 50 mbar,preferably 10 to 30 mbar, the bottom temperatures being 210 to 250° C.,preferably 220 to 235° C. Under these conditions the TMP distils offtogether with other compounds more volatile than TMP, i.e. the so-calledlow-boiling components. In connection with the present invention, it isimportant that the high temperatures at the bottom of the column causethe cleavage of some of the high-boiling components which are TMPderivatives, examples being dimethylolbutyraldehyde TMP acetal andhigher acetals. The decomposition of the high-boiling components can beaccelerated by suitable measures known per se, for example the additionof acid. This makes it possible to increase the yield of TMP evenfurther. It is particularly advantageous to add acid according to theprocess described in the German patent application entitled “Verfahrenzum Zersetzen von bei der Synthese mehrwertiger Alkohole gebildeterhochsiedender Nebenprodukte” [“Process for decomposing high-boilingby-products formed in the synthesis of polyhydric alcohols”], referenceno. 199 63 437.8 (Applicant: BASF AG). This gives a bottom productcontaining 1 to 50% by weight of TMP. In process step (c), thelow-boiling components and the TMP are distilled together from theimpurities remaining in the bottom product, and are collected. Thedistillation is generally carried out by means of a column with a refluxratio of 0 to 3, preferably 0 to 1. The conventional internals known tothose skilled in the art, preferably regularly spaced packings, are usedin the column.

[0041] The distillate originating from step (c), which contains TMP andthe so-called low-boiling components, for example 2-ethylpropanediol orTMP formate, is then purified by distillation in step (d). Thisdistillation is generally carried out in a column. The low-boilingcomponents are separated off at the top and the TMP is withdrawn fromthe column as a side discharge, preferably below the feed. The sidedischarge can be liquid, although it is preferably gaseous.

[0042] A TMP with a purity of >99% and an APHA color index of 20 to 200can be obtained by this method. The distillation is carried out in theconventional columns known to those skilled in the art, preferablycolumns equipped with internals. Regularly spaced packings arepreferably used. The distillation is carried out at pressures of 10 to40 mbar, preferably 20 to 30 mbar. It has proved advantageous to extracta small stream from the bottom of the column in order to prevent anaccumulation of high-boiling and/or colorizing components.

[0043] In the variant of the present invention, steps (b) and (d) can becombined. In this case the mixture obtained after hydrogenation in step(a) is distilled in such a way that not only the very readily volatilecompounds, such as water, methanol and triethylamine, but also the otherso-called low-boiling components are distilled off, which in principle,as described previously, are not removed until after the high-boilingcomponents have been separated from the TMP. Examples of saidlow-boiling components are 2-ethylpropanediol or TMP formates, althoughoften some of these have also already passed over with the very readilyvolatile compounds. To achieve this complete separation of the compoundsmore volatile than TMP, said separation is carried out at pressures of10 to 40 mbar and temperatures of 170 to 210° C. It can be carried outin evaporators, but it is preferable to use distillation columns becausethe distillation of TMP can be prevented in this way.

[0044] In another variant of the invention, steps (c) and (d), i.e. thejoint distillative separation of TMP and low-boiling components from thehigh-boiling components and the subsequent distillative separation ofthe low-boiling components from TMP, can be carried out in a singlestep. To do this, when the mixture withdrawn from step (b) is heatedunder the abovementioned conditions, the volatile compounds have to bedistilled off over a column of appropriate separation efficiency andseparated with said column. It is advantageous to use columns with aside discharge, i.e. ones which are also used when carrying out step (d)separately from step (c).

[0045] The TMP withdrawn from step (d), i.e. from the purification bydistillation to separate off the low-boiling components, can besubjected to a second purification by distillation (e). This seconddistillation is optional and serves to improve the color index in caseswhere it is desired to obtain, as far as possible, a colorless TMP. Itmay be said that the distillation (d) is carried out in order to obtaina pure product. The distillation (e) affords virtually no furtherimprovement in the purity, only in the color index.

[0046] The distillation is generally carried out in a column.Lower-boiling colorizing components are separated off at the top and theTMP is withdrawn from the column as a side discharge, preferably belowthe feed. The side discharge can be liquid, although it is preferablygaseous.

[0047] This procedure makes it possible to obtain a TMP with colorindices of 10 to 100 APHA. The distillation is performed in thecustomary columns known to those skilled in the art, preferably incolumns equipped with internals. The preferred internals are regularlyspaced packings. The distillation is carried out at pressures of 5 to 40mbar, preferably 20 to 30 mbar. It has proved advantageous here toextract a small stream from the bottom of the column so as to prevent anaccumulation of higher-boiling and/or colorizing components.

[0048] The process according to the invention will now be explained withreference to the diagram shown in FIG. 1, which illustrates oneparticular variant of the process.

[0049] The crude TMP solution 1 obtained after the condensation reactionand subsequent hydrogenation is introduced into the low-boiling column2, in which a mixture 3 of water and low-boiling components, such asmethanol or trialkylamine, is separated off. After purification, thisdialkylamine can be re-used as a catalyst in the condensation reactionbetween n-butyraldehyde and formaldehyde. The bottom product 4 withdrawnfrom the low-boiling column, which contains TMP, high-boiling componentsand the low-boiling components not separated off in the column 2, isintroduced into the reactor 6, in which trimethylolpropane formates arecleaved. This is effected by adding a dialkylamine or an alcohol, forinstance methanol, denoted by the number 5. The conversion of the TMPformates to TMP and either formates of the alcohol used or formamides ofthe amine used takes place in the reactor 6. The use of the reactor 6for cleavage of the formates is optional. This reactor is not used ifthe conditions in the low-boiling column 2 are chosen so that no TMPformates are produced from trialkylammonium formate and free TMP.

[0050] The solution 7, depleted in TMP formates, now passes into thehigh-boiling separator 8, in which the solution 7 is heated underreduced pressure to temperatures at which a mixture 9 of TMP and morevolatile substances is distilled off. At the same time, high-boilingcomponents which are TMP derivatives are decomposed by the hightemperatures. These then also distil off with the mixture 9 and increasethe yield of TMP. This leaves a bottom product 10 rich in high-boilingcomponents. Said bottom product 10 is discarded or can be combusted toraise superheated steam. It can also be distilled further in order topurify particular compounds.

[0051] The mixture 9 of TMP and low-boiling components is thenintroduced into the first distillative purification unit 11, where theimpurities 12 boiling below TMP are separated off at the top. They canbe discarded, combusted to raise superheated steam or purified furtherto recover individual components contained therein. A stream 13 ofcolorizing and high-boiling components is extracted from thedistillative purification unit 11 and can be recycled into thehigh-boiling separator 8 or else discarded. Finally, the pure TMP 14withdrawn from the unit 11 is subjected to an (optional) distillation ina color index distillation device 15, where the lower-boiling colorizingcomponents 16 are distilled off at the top. They can be discarded orrecycled into the distillative purification unit 11. High-boiling andcolorizing components 17 which accumulate slowly at the bottom of thedevice 15 are extracted and discarded or recycled into the distillativepurification unit. A pure TMP 18 with a low color index is recovered.

[0052] The present invention will now be illustrated with the aid of theexamples below. The trimethylolpropane used in all the examples had beenprepared as follows:

[0053] An apparatus consisting of two heatable stirred tanks with anoverall capacity of 72 l, interconnected by overflow tubes, was chargedcontinuously with fresh aqueous formaldehyde solution (4300 g/h in theform of a 40% aqueous solution) and n-butyraldehyde (1800 g/h), and withfresh trimethylamine as catalyst (130 g/h) in the form of a 45% aqueoussolution. The reactors were heated to a constant temperature of 40° C.

[0054] The discharge was passed directly into the top of a falling filmevaporator with attached column (superheated steam at 11 bar), where itwas separated by distillation under atmospheric pressure into alow-boiling top product, essentially containing n-butyraldehyde,ethylacrolein, formaldehyde, water and trimethylamine, and ahigh-boiling bottom product.

[0055] The top product was continuously condensed and recycled into thereactors described above.

[0056] The high-boiling bottom product from the evaporator (approx. 33.5kg/h) was treated continuously with fresh trimethylamine catalyst (50g/h, in the form of a 45% aqueous solution) and transferred to aheatable, packed tubular reactor with an empty volume of 12 l. Thereactor was heated to a constant temperature of 40° C.

[0057] The discharge from the secondary reactor was passed continuouslyinto the top of another distillation device for separation of theformaldehyde (superheated steam at 11 bar), where it was separated bydistillation into a low-boiling top product, essentially containingethylacrolein, formaldehyde, water and trimethylamine, and ahigh-boiling bottom product. The low-boiling top product (27 kg/h) wascontinuously condensed and recycled into the first stirred tank, whilethe high-boiling bottom product was collected.

[0058] In addition to water, the resulting bottom product containedessentially dimethylolbutyraldehyde, formaldehyde and traces ofmonomethylbutyraldehyde. This bottom product was then subjected tocontinuous hydrogenation. This was done by hydrogenating the reactionsolution at 90 bar and 115° C. in a primary reactor by the loop/tricklemethod and in a downstream secondary reactor by the loop method. Thecatalyst was prepared analogously to D of DE 198 09 418. It contained24% of CuO, 20% of Cu and 46% of TiO2. The apparatus used consisted of aheated primary reactor with a length of 10 m (internal diameter: 27 mm)and a heated secondary reactor with a length of 5.3 m (internaldiameter: 25 mm). The loop throughput was 25 l/h of liquid and thereactor feed was adjusted to 4 kg/h, corresponding to a hydrogenationdischarge of 4 kg/h.

EXAMPLE 1

[0059] The TMP used had a composition of 22.6% by weight of TMP, 1.4% byweight of methanol, 2.1% by weight of trimethylammonium formate, 1.1% byweight of methylbutanol, 0.7% by weight of ethylpropanediol, 1.2% byweight of adducts of TMP with formaldehyde and methanol, <0.1% of TMPformate, 1.2% by weight of TMP dimethylbutanal acetals, 2.9% by weightof high-boiling components and 66.2% by weight of water. 5 kg/h of thiscrude solution were worked up. The crude mixture was first dehydrated inthe low-boiling column at 400 mbar and a bottom temperature of 160° C.,the feed entering the middle of the column. The reflux ratio wasadjusted to 0.3. 1.3 kg/h of a mixture consisting of 83% by weight ofTMP, higher-boiling impurities, approx. 1% by weight of water and 7.5%by weight of TMP formates were withdrawn from the bottom of the columnand transferred to the high-boiling separator. This consisted of acolumn operated at 20 mbar and a reflux ratio of 0.5. High-boilingcomponents containing 24.4% by weight of TMP were thus separated off atthe bottom of the column. The TMP freed of high-boiling components iswithdrawn from the top and purified in a distillation unit consisting ofa column with a side discharge point, operated at 20 mbar withtemperature-regulated reflux. The components boiling below TMP, such as2-ethylpropanediol and TMP formate (20.7% by weight), are distilled offat the top, the distillate having a residual TMP content of 6.6% byweight. To prevent the accumulation of high-boiling components, 150 g/hare withdrawn from the bottom of the column and recycled into thehigh-boiling separation stage. Pure TMP with a content of >99% iswithdrawn from the vaporous side discharge above the evaporator in anamount of 1030 g/h. The overall yield of the work-up is 90%.

EXAMPLE 2

[0060] TMP was prepared as described in Example 1 and had a compositionof 22.1% by weight of TMP, 0.4% by weight of methanol, 1.5% by weight oftrimethylammonium formate, 0.7% by weight of methylbutanol, 0.5% byweight of ethylpropanediol, 1.2% by weight of adducts of TMP withformaldehyde and methanol, <0.1% by weight of TMP formate, 1.4% byweight of acetals of TMP with dimethylbutanal, 2.2% by weight ofhigh-boiling components and 69.9% by weight of water. The throughput was4 kg/h. The crude mixture was first introduced into the low-boilingcolumn, dehydration being carried out at 400 mbar and 180° C. The feedentered the middle of the column. The reflux ratio was adjusted to 0.3.1.1 kg/h of dehydrated TMP, containing 82.8% by weight of TMP,higher-boiling impurities, approx. 0.5% by weight of water and 6.6% byweight of TMP formate, were withdrawn from the bottom of the column,mixed with 40 g/h of dimethylamine and introduced into a tubular reactorin which the TMP formate is reacted with dimethylamine at 120° C. andwith a residence time of 1 hour to give TMP and dimethylformamide. Theresulting conversion was 95%, making it possible to lower the residualcontent of TMP formate to <0.3% by weight. The reaction dischargeobtained was introduced into the high-boiling separation device, wherethe high-boiling components were separated off at 30 mbar and a refluxratio of 0. High-boiling components with a residual TMP content of 10%by weight were thus separated off at the bottom of the column. Duringthe separation, 85% phosphoric acid is introduced batchwise into thebottom of the column to give a phosphoric acid concentration of between100 ppm and 1000 ppm. TMP freed of high-boiling components is distilledoff at the top and then purified by distillation carried out at 30 mbarand with temperature-regulated reflux in a column with a side dischargepoint. In this distillative purification, compounds boiling below TMP,such as 2-ethylpropanediol or TMP formate, are distilled off at the topto give a top product with a residual TMP content of 7% by weight. Adischarge with a TMP content of >98% by weight is withdrawn from thebottom of the column in an amount of 150 g/h and recycled into thehigh-boiling separator. Pure TMP with a content of >99% is withdrawnfrom the vaporous side discharge above the evaporator. The overall yieldof the work-up is 98%, the TMP obtained having color indices of between30 and 150 APHA.

EXAMPLE 3

[0061] The procedure was as described in Example 2. To improve the colorindex, the pure TMP obtained was then subjected to a furtherdistillation in a column with a side discharge point. This color indexdistillation was carried out at 20 mbar and a reflux ratio of 35. 150g/h of bottom discharge with a TMP content of >98% and 30 g/h of topdischarge with a TMP content of >98% were recycled into the distillativepurification stage. A pure colorless TMP with a color index of 15 to 50APHA was withdrawn from the vaporous side discharge above theevaporator.

EXAMPLE 4

[0062] The TMP used had a composition of 1.59% of trimethylammoniumformate, 27.0% of trimethylolpropane and 69% of water, the remainderbeing by-products, and was free of trimethylolpropane formate. This TMP(3500 g) was then pumped at 50 mbar and a rate of 600 ml/h into a Sambayevaporator at a Sambay temperature of 180° C. The bottom and topproducts were withdrawn. The bottom discharge was taken from a recyclestream of 6.2 l/h. The bottom product (993 g) had a composition of 92.2%of TMP, 0.31% of trimethylammonium formate, 0.55% of trimethylolpropaneformate and 0.49% of water, the remainder consisting of otherby-products. This bottom product can be worked up further as describedin one of Examples 1 to 3. The top product (2498 g) had a composition of2.1% of trimethylammonium formate, 2.1% of TMP and 95.9% of water andwas free of trimethylolpropane formate. 94% of the trimethylammoniumformate could thus be separated off at the top.

We claim:
 1. A process for removing trialkylammonium formate frompolyhydric alcohols obtained by condensing formaldehyde with a higheraldehyde, wherein the trialkylammonium formate, together with methanol,trialkylamine and optionally water, is distilled from the alcohol atbottom temperatures of <200° C., preferably 60 to 140° C. andparticularly preferably 80 to 120° C., at pressures of <400 mbar,preferably 20 to 200 mbar and particularly preferably 40 to 150 mbar,and with short residence times.
 2. A process as claimed in claim 1,wherein the polyhydric alcohol has been obtained by the hydrogenation ofalkylolated alkanal, preferably by the hydrogenation ofdimethylolbutanal to trimethylolpropane.
 3. A process as claimed inclaim 1 or 2, wherein, in a separate step, the water is separated offfirst, followed by the other impurities, including tri-alkylammoniumformate.
 4. A process for the purification, by distillation, oftrimethylolpropane originating from the hydrogenation of2,2-dimethylolbutanal, said process comprising the following steps: (a)reaction of n-butyraldehyde with formaldehyde in the presence ofcatalytic amounts of a tertiary amine, and hydrogenation of theresulting mixture to give a mixture containing trimethylolpropane; (b)separation of water, methanol, trialkylamine and/or trialkylammoniumformate by distillation; (c) heating of the residue obtained in (b)under reduced pressure to a temperature at which TMP is volatile andcompounds boiling above TMP are cleaved, in order to separate off, bydistillation, TMP and compounds more volatile than TMP; (d) distillationof the distillate obtained in (c) in order to separate off the morevolatile compounds and recover pure TMP; and (e) optional distillationof the trimethylolpropane obtained in (d) in order to recover TMP with alow APHA color index.
 5. A process as claimed in claim 4, wherein thedistillative separation of step (b) is carried out at pressures of <400mbar, preferably 20 to 200 mbar and particularly preferably 40 to 150mbar, at bottom temperatures of <200° C., preferably 60 to 140° C. andparticularly preferably 80 to 120° C., and with short residence times,in such a way that at most small amounts of TMP react withtrialkylammonium formate formed in step (a) to give TMP formates andtrialkylamine.
 6. A process as claimed in claim 4 or 5, wherein step (b)is subdivided into the following steps: (bi) separate separation ofwater; and (bii) distillation of the other impurities, includingtrialkylammonium formate.
 7. A process as claimed in claim 4, whereinstep (b) is carried out at pressures of >200 mbar, preferably >400 mbar,at bottom temperatures of >140° C., preferably 160 to 180° C., and withlong residence times, in such a way that at least the bulk of the TMPreacts with trialkylammonium formate formed in step (a) to give TMPformates and trialkylamine.
 8. A process as claimed in claim 7, whereinstep (b) is followed by a step (bb) in which the TMP formates producedare converted, by reaction with a lower alcohol, preferably methanol, ora secondary amine, preferably a dialkylamine, to TMP and the formate ofthe alcohol used or the formamide of the amine used.
 9. A process asclaimed in any of claims 4 to 8, wherein step (c) is carried out atbottom temperatures of 210 to 250° C., preferably 220 to 235° C., andpressures of 5 to 50 mbar, preferably 10 to 30 mbar.
 10. A process asclaimed in any of claims 4 to 9, wherein step (d) is carried out atbottom temperatures of 170 to 210° C., preferably 180 to 200° C., andpressures of 10 to 40 mbar, preferably 20 to 30 mbar.
 11. A process asclaimed in any of claims 4 to 10, wherein step (e) is carried out atbottom temperatures of 170 to 210° C., preferably 180 to 200° C., andpressures of 5 to 30 mbar, preferably 20 to 30 mbar.
 12. A process asclaimed in any of claims 4 to 11, wherein steps (b) and (d) are combinedso that the mixture originating from step (a) is distilled in such a waythat all the compounds more volatile than TMP distil off and pure TMP isrecovered in step (c) by distillative separation of the high-boilingcomponents.
 13. A process as claimed in any of claims 4 to 11, whereinsteps (c) and (d) are combined so that the separation of thehigh-boiling components remaining in the bottom of the column, thedistillation of compounds more volatile than TMP, and the recovery ofpure TMP, are carried out in one step using a column of appropriateseparation efficiency, preferably a column with a side discharge.
 14. Aprocess as claimed in any of claims 4 to 13, wherein step (e) is carriedout in order to remove colorizing components.